In wired communication networks, signal energy is mostly confined in a physical medium, such as conductive wires or optical fibers. Hence, signals can only be accessed by physically attaching to the medium.
In wireless networks, any receiver within range of the transmitter can intercept the signals. Therefore, conventionally secure communication typically uses cryptography and asymmetric public and private keys at the transmitter and the receiver. A public key infrastructure (PKI) generates, distributes and maintains the public keys, in which a trusted certificate authority (CA) binds all the public keys with respective user identity and issues a public key certificate to the respective user. In order to establish secure communication, the transmitter first verifies the receiver's public key certificate. After the public key is verified, messages are encrypted using the receiver's public key, and the messages can only be decrypted using the corresponding private key. Generation of public keys requires significant computational overhead.
For many wireless networks, such as ad hoc network, access to a PKI is difficult, or unavailable. Wireless nodes do not have the computational power to generate public keys either. In such cases, security communication in such wireless networks becomes a challenge. Given this, realizing security in wireless communication networks is of great interest.
Recently, physical layer security has been investigated for wireless networks. Based on information theory, messages transmitted at bit rates higher than a channel capacity cannot be decoded correctly. It is therefore possible to transmit a message to intended users securely, providing that the channels between the transmitter and intended receivers have higher capacity than channels between the transmitter and eavesdroppers. However, in practice, it is difficult to guarantee that such a condition is satisfied.
Another approach generates secret session keys in a wireless node. The reciprocity of wireless channels enables two nodes to generate a pair of secret keys that are made identical by quantizing parameters of the channel. After a matching pair of keys are generated by each node, the keys can be used to encrypt messages between the nodes. Because eavesdroppers have wireless channels that are different than the two nodes, the eavesdroppers cannot produce the same keys, and the secure communication is guaranteed. For that approach, it is essential that the independently generated keys match completely. However, due to the noise, interference and hardware impairment, it is not always guaranteed that the keys generated by a pair of wireless nodes are exactly the same.
Low-density parity check (LDPC) codes can be used for forward error correction (FEC) codes, and are widely used to reduce channel noise and key mismatches. Given the channel statistics, one can design good LDPC codes that perform very closely to the channel capacity. However, in reality, channel parameters cannot always be obtained accurately. Moreover, the channel can be time-variant. Therefore the code rate should be determined dynamically.